Reading this editorial over at Glitchy Tasty you can learn how Fallout: New Vegas succeeded in achieving a lot of immersion.

The second major way New Vegas covers agency is through its quest and reputation systems. A large variety of the quests throughout the game are multi-linear, and can end in multiple ways. The choices made by the player involving how to go about each quest will directly result in how various factions react toward your character. This means that you can entirely change the games ending, as well as the course leading up to it depending on how you want it to go. For instance, you may be doing a quest to wipe out a large gang, but rather than eliminating the gang, you could have the option of instead joining them and attacking the militia instead. By joining the gang, you will ruin your reputation with that militia, forever changing how they act towards you for the remainder of the game. This gameplay mechanic was certainly present in Fallout 3, though it has really been taken to the next level with New Vegas, offering considerably more branching opportunities.